Ziegler-Natta type catalysts are well known and have been used since the early 1950's. Generally the catalyst comprises a transition metal compound, typically titanium in the 3 or 4 valence state (e.g. TiCl.sub.3 or TiCl.sub.4) together with at least one activator, typically an aluminum compound such as a trialkyl aluminum (e.g. triethylaluminum (TEAL)) or an alkyl aluminum halide (e.g. diethylaluminum chloride (DEAC)) and the like. The catalysts may contain magnesium chloride which may be generated from a dialkyl magnesium compound and active chloride. Typically, the catalysts are prepared in conjunction with an electron donor which tends to narrow the molecular weight distribution in the resulting polymer. A good description of these types of catalysts is contained in U.S. Pat. No. 4,302,566 issued Nov. 24, 1981 to Karol et al., assigned to Union Carbide Corporation. Karol teaches that it is essential to use an electron donor in the preparation of the catalyst. The use of an electron donor has been eliminated from the catalysts of the present invention.
U.S. Pat. No. 5,633,419 issued May 27, 1997 to The Dow Chemical Company discloses a supported Ziegler-Natta type catalyst or catalyst precursor. However, it is a limitation of the reference that the halide is used in the form of a hydrogen halide (e.g. HCl). The reference teaches away from the subject matter of the present invention which is limited to the use of alkyl halides as the halide source. Additionally, the reference appears to teach the use of a support such as silica, which has been treated either thermally or chemically, but not both, which is contrary to the method for preparing the support of the present invention.
European Patent Application 0 744 416 published Nov. 27, 1996 in the name of BP Chemicals Limited (BP) teaches supported catalysts and catalyst precursors. The reference teaches that the support is reacted with either the aluminum compound or the magnesium compound but not a concurrent treatment with a mixture of aluminum and magnesium (e.g. MAGALA, a mixture sold by Akzo). The reference teaches against the addition of a second aluminum compound. Additionally, in view of the examples, the BP patent strongly suggests that an electron donor needs to be used in the preparation of the catalyst, a feature that has been eliminated from the present invention. This reference also teaches that the halogenating agent must be used in a molar ratio such that all of the Mg and Al compounds are converted to their respective halides, a limitation which is not put on the present invention. That is, in accordance with the present invention, there is an excess of aluminum and magnesium over reactive halide so that not all of the aluminum and magnesium is precipitated.
BP patent EP 0 595 574 B1 granted in Jan. 2, 1997 requires that the catalyst be prepared on a support treated with an organosilicon compound. The supports used in accordance with the present invention are treated thermally and chemically (aluminum alkyls) but not with organosilicon.
BP patent EP 0 453 088 B1 granted in Jul. 31, 1997 teaches that the catalyst be made on dehydrated silica and that the catalyst needs to be pre-polymerized to obtain a useful catalyst for the synthesis of linear low density polyethylene (LLDPE). The catalysts of the present invention do not need to be pre-polymerized to be useful for the polymerization of ethylene to LLDPE. The patent teaches about washing the catalyst to remove excess Mg compounds which is not relevant to our catalyst. This patent also teaches about the need for high Ti:(Mg+Al) ratios, a restriction which is removed in the present invention.
The present invention seeks to provide a supported Ziegler-Natta type catalyst which is preferably prepared in the absence of an electron donor, and provides a polymer having a broad molecular weight distribution. The present invention relates to supported Ziegler-Natta catalysts which are useful in olefin polymerization, and more particularly in the fluidized gas phase polymerization of alpha-olefins.